It may just be time for them to replace some media in their filter. Do you have a tds meter? If it reads close to 0, then I'd go ahead and test your phos meter for accuracy. But if you get a higher reading for tds...it's prob not your meter that's off. Last time I picked up water there, which was a few weeks ago, it read 023 on my tds meter. I decided to top off anyway and had some diatoms pop up that week.
Not certain that I can say that's to blame...but I'd suspect there could be some phosphate in the water if it's gone this long unchecked.